1. Field of the Invention
This invention pertains to a temperature indicating label wherein portions of the irreversibly fusible material are pre-fused in order to create an in situ standard or visible pattern. When the label is thereafter exposed to the threshold temperature, the label becomes uniformly fused and the pattern disappears.
2. Description of the Prior Art
Temperature indicating labels, utilizing irreversibly fusible materials, are widely available and produced by several manufacturers. For the most part, they follow the early design of Wahl (U.S. Pat. No. 3,002,385) who laminated fusible materials in an absorptive setting so that once melting occurred a color change took place. In the examples given in the ""385 patent the absorptive materials were carbon black, porous paper, or a combination of the two. The change in color was produced by the exposure of a previously hidden object (colored paper) which became visible after melting of the selected fusible masking coating.
As disclosed in the ""385 patent, several materials with different melting points could be placed adjacent to one another, so long as they were not in contact, to produce a temperature indicator capable of recording the extent of heating over a range of temperatures. For example, adjacent areas of the device could have materials melting at 100xc2x0 F., 110xc2x0 F., 120xc2x0 F. and 130xc2x0 F. If the device reached a temperature of 105xc2x0 F. only the first (100xc2x0 F.) region would change color. If the first three regions changed color one could assume that the indicator had been to exposed to a temperature greater than 120xc2x0 F., but less than 130xc2x0 F.
One advantage this embodiment had over the prior art (Loconti, U.S. Pat. No. 2,928,791) which focused on compositions of matter, was the construction of a label that could be adhered to another object and left in place for an unspecified period of time.
But it these two advantages (multiplicity of temperatures and unspecified holding time) that have subsequently demonstrated deficiencies in the devices as presented today. Indeed this deficiency can be made clear even when multiple temperatures are not presented on the same temperature indicating label.
The utility of an irreversible temperature indicator depends on being able to clearly differentiate between xe2x80x9cbeforexe2x80x9d and xe2x80x9cafterxe2x80x9d states of the indicator. That is, one should be able to easily discern the difference between an indicator which contains material that has not passed through a fused state and returned to its solid condition, and an indicator which has fused, and then re-solidified.
The most common form of the indicator label uses a thin coating of fusible opaque material over a colored background. For simplicity, the xe2x80x9cbeforexe2x80x9d state can be characterized as xe2x80x9cwhitexe2x80x9d while the xe2x80x9cafterxe2x80x9d state is xe2x80x9cblackxe2x80x9d. But in practice the initial whiteness depends on many factors including the refractive index of the fusible material, its particle size, the nature of binders and additives used in the coating. The porosity and optical properties of the substrate also contribute to the initial appearance.
This means that there are often lot-to-lot variations in the appearance and performance of any single temperature indicator, which can lead to confusion in the interpretation of the state of the indicator (xe2x80x9cbeforexe2x80x9d versus xe2x80x9cafterxe2x80x9d). This degree of uncertainty is particularly high when objects marked with indicators produced at different times are in close proximity to one another. Should one indicator appear less white than that on an adjacent article the observer may be led to conclude that the article has been exposed to a temperature beyond the rating of the label. This xe2x80x9cfalse positivexe2x80x9d may lead to costly and erroneous conclusions regarding the state of the object whose temperature must be clearly known.
The opportunity for erroneous conclusions is further enhanced in the case of multiple temperatures being represented on the single indicating label. For example, in the case cited earlier (100xc2x0 F., 110xc2x0 F., 120xc2x0 F., and 130xc2x0 F.) it is entirely reasonable that the 130xc2x0 F. portion of the label would be initially less white than the regions comprising the three other temperatures. The conclusion that the label had reached 130xc2x0 F. may, or may not be correct.
If the object were uniformly heated it is unreasonable to conclude that the 130xc2x0 F. indicator could signal, leaving the lower temperature indicators unchanged. But the possibility for non-uniform heating also exists, and a reasonable conclusion may be that only the region directly under the 130xc2x0 F. indicator region did surpass the temperature, leaving the other regions unchanged.
There is yet another deficiency in these products, unrecognized at the time of their first invention and use. That deficiency, the possible sublimation of the fusible coating, is most clearly observed in applications where the temperature indicating label is held for long periods of time at a temperature below the melting point of the fusible compound.
For example, Loconti, in the ""791 patent, includes adipic acid in a listing of white crystalline pigments suitable for his invention. The Loconti reference gives the melting point as 135xc2x0 C.-154xc2x0 C. and the transition temperature as 146xc2x0 C. Yet far below these temperatures (86xc2x0 C. to 133xc2x0 C. as cited by the Handbook of Chemistry and Physics in the section xe2x80x9cSublimation Pressure for Organic compounds) the material exhibits a significant vapor pressure, which will allow it to sublime.
This will lead to the gradual disappearance of the material from the label surface. Over time this disappearance will lead to darkening, and may finally lead to a situation where the formerly white region becomes indistinguishable from a black region. This means that the signal area can change from xe2x80x9cbeforexe2x80x9d to xe2x80x9cafterxe2x80x9d without ever having reached the designated melting point of the material.
The rate at which this transition occurs depends on multiple factors such as coating thickness and label construction as well as the ambient temperature and pressure surrounding the temperature indicating label. If sublimation is unrecognized this phenomenon can lead to faulty interpretation and costly false-positive situations.
As a partial aid to signal interpretation some manufacturers provide an external standard as part of the temperature indicating label. One example is the presence of a printed black circle surrounding the white temperature-responsive area. The intuitive response of the user is to match the appearance of the printed circle with the appearance of the temperature-responsive area within. But because of the nature of the printing process, and the nature of the two surfaces being compared (the temperature responsive surface will be matte, the printed black surface will be glossy) the external standard can only be a poor approximation of congruent final states.
Other prior art includes U.S. Pat. No. 5,997,927 entitled xe2x80x9cIndicator, An Associated Package and Associated Methodsxe2x80x9d issued on Dec. 7, 1999 to Gics, wherein an indicator scale is printed on the package to permit a user to compare the indicator scale to the label to determine if the fusing has occurred which indicates that the threshold temperature has been reached. This, however, still provides a complicated label which is not intuitive, and may not provide a clear indication of whether or not fusing has occurred if there are variations in the printing of the indicator scale or if sublimation has occurred.
Other prior art includes U.S. Pat. No. 6,042,264 entitled xe2x80x9cTime-Temperature Indicator Device and Method of Manufacturexe2x80x9d issued on Mar. 28, 2000 to Prusik et al.; U.S. Pat. No. 5,912,204 entitled xe2x80x9cThermosensitive Recording Adhesive Labelxe2x80x9d, issued on Jun. 15, 1999 to Yamada et al.; U.S. Pat. No. 5,795,065 entitled xe2x80x9cTemperature-Time-Pressure Detectorxe2x80x9d issued on Aug. 18, 1998 to Barham; U.S. Pat. No. 5,709,742 entitled xe2x80x9cTime-Temperature Indicator Device and Method of Manufacturexe2x80x9d issued on Jan. 20, 1998 to Prusik et al.; U.S. Pat. No. 5,476,792 entitled xe2x80x9cTime-Temperature Indicator Devicesxe2x80x9d issued on Dec. 19, 1995 to Ezrielev et al.; U.S. Pat. No. 5,188,457 entitled xe2x80x9cMeasurement of the Maximum Temperature Attained by an Articlexe2x80x9d issued on Feb. 23, 1993 to O""Hara; U.S. Pat. No. 4,929,090 entitled xe2x80x9cTemperature History Indicating Labelxe2x80x9d issued on May 29, 1990 to Grahm.
It is therefore an object of the present invention to provide a label which indicates that a maximum temperature has been exceeded.
It is therefore a further object of the present invention to provide a label which quickly and intuitively indicates to a user that a maximum temperature has been exceeded.
It is therefore a still further object of the present invention to provide a label which unambiguously indicates to a user that a maximum temperature has been exceeded.
It is therefore a still further object of the present invention to provide a label which will continue to be functional for at least some period in the event of sublimation of the temperature indicating material.
It is therefore a still further object of the present invention to provide a label which may be used after some sublimation has occurred, without reaching a xe2x80x9cfalse positivexe2x80x9d state.
These and other objects are attained by providing a label with irreversibly fusible material which permanently fuses and changes color after exposure to a selected threshold temperature. The label, however, during manufacture, is initially selectively fused in situ in order to create a contrasting pattern between the selectively fused and the unfused portions. This initial selective fusing may be performed by direct contact with a heated surface, such as hot stamping, or by radiant energy from an infra-red lamp. Other methods may be used to create the contrasting pattern, in situ, between the fused and unfused portions.
Alternatively, incompletely obscuring the functional surface of the indicator may be achieved by printed a pattern of fusible material on an absorptive surface which allows a portion of the surface to remain visible. Subsequent melting and fusing of the fusible material would result in the unobscured view of the functional surface of the indicator.
A further alternative is to place a fully coated surface in proximity to wholly uncoated surface during construction of the label. For example, white (coated) and black (uncoated) semicircles could be joined to give the appearance of a half white and half black circle. Subsequent fusing of the coated surface would result in a uniform black appearance.